type-system: Add T? sugar for Option<T>

Status

T? is not part of the first stable Silk language. Stable source imports and writes
Option<T> explicitly. This issue tracks post-stable syntax sugar for the same ordinary nominal
type; it must not introduce a privileged optional type.

Proposed surface

import silk.option { Option }

fn findUser(id: i32) -> User? {
  // ...
}

Within this module, User? denotes the same type as Option<User>.

Required behavior

• Option must resolve through ordinary lexical scope. Source must import it or declare another
  visible binding according to the normal name-resolution rules.
• Unknown, ambiguous, private, or shadowed Option bindings receive the ordinary diagnostics.
• The compiler must not recognize silk.option or any standard-library declaration by spelling.
• T? and Option<T> are the same semantic type after resolution and have identical ownership,
  layout, MIR, and backend behavior.
• The formatter preserves the source spelling rather than rewriting one form into the other.
• Hover, completion, navigation, and documentation may explain the resolved Option<T> identity
  while retaining the written T? span.

Examples

Valid when Option is in scope:

import silk.option { Option }

fn absent() -> i32? {
  return Option.none<i32>()
}

Invalid when no Option binding resolves:

fn absent() -> i32? {
  return 0
}

The second program must report the ordinary unresolved-type dependency rather than silently loading
the standard library.

Out of scope

• Null values or nullability.
• Optional chaining and null-coalescing operators.
• Compiler privilege for the standard library.
• Changing the representation of Option<T>.

Acceptance criteria

• T? resolves to the same semantic type as an in-scope Option<T>.
• Missing, ambiguous, private, and shadowed bindings use ordinary resolution diagnostics.
• Formatter and LSP presentation preserve the authored sugar.
• Ownership, layout, MIR, evaluator, native LLVM, and direct Wasm match explicit Option<T>.
• No compiler phase recognizes a standard-library declaration by spelling.

No files or tests are named. Start by tracing type parsing and ordinary name resolution, then follow explicit Option through ownership, layout, MIR, evaluator, LLVM, Wasm, formatter, and LSP handling. Done means T? matches explicit Option without spelling-based standard-library treatment, while authored syntax and ordinary diagnostics are preserved.